The present invention relates in general to the reception and decoding of television signals containing closed caption information. More particularly, it relates to an improved circuit and method for data slicing an analog closed caption signal to produce an optimum form of the digital closed-caption information contained therein.
In North America, a television (TV) receiver having a 13-inch or larger screen is typically equipped with circuitry for superimposing text captions of speech on the television picture. This is known as a caption system and allows hearing impaired viewers to read the audio portion of a television program. The United States has proposed regulations that would require the provision of a caption decoder in all 13 inch or larger TV receivers sold in the U.S., even if the unit is not actually manufactured in the U.S.
Most caption decoders currently available in the U.S. are discrete teletext receivers or adaptors that must be connected to the TV receiver. A caption decoder installed in a TV receiver is often referred to as a closed-caption decoder, and caption data or signals multiplexed with television signals are often referred to as closed caption (CC). In order to remain competitive, manufacturers and/or retailers rarely charge extra for TV receivers with on-board closed-caption decoders. Accordingly, there is always a need for a closed caption decoder that is effective, simple, and inexpensive to manufacture.
According to current standards, composite TV signals include a vertical sync signal, a horizontal sync signal, and a closed caption signal. FIGS. 7a to 7d illustrate examples of a composite TV signal (with closed caption), a vertical sync signal, a horizontal sync signal, and a closed caption signal, respectively. The closed-caption signal is typically inserted in the twenty-first period of the horizontal sync signal (FIG. 7c) as measured from the decay edge (e) of the vertical sync signal (FIG. 7b). Such a closed-caption signal is often referred to as Field 1, while a closed caption signal in the 284th horizontal sync period (not shown) is often referred to as Field 2.
The closed-caption signal shown in FIG. 7d includes a 3.58 MHz burst signal (.apprxeq.10.5 .mu.s from the decay of the horizontal sync signal to the end of the burst), a 7-cycle clock run-in signal (.apprxeq.12.91 .mu.s from the end of the burst signal, the frequency being 32.times.fH where fH is 15.75 KHz of the horizontal sync signal frequency), a start bit signal (.apprxeq.6.958 .mu.s from the end of the clock run-in signal) indicating the start of caption data, and a data character signal (.apprxeq.31.778 .mu.s from the end of the start bit signal) containing 2 bytes of caption data.
Each of the 2 bytes of closed caption data has its uppermost (or leftmost) bit assigned as a parity bit. The parity bit is used for the detection of data error. In an odd-parity system, a bit is added to the transmitted bits (0s and 1s) so that there are an odd number of 1s, and the data bits are judged to be correct if there are an odd number of 1s. If the number of 1s is even, the data bits are judged to be abnormal and are not accepted.
When analog signals are converted to digital signals, a threshold level is often used to determine whether the input analog signal will be interpreted as a logical 0 or a logical 1. In general, if the input analog signal exceeds the desired threshold level, the input analog signal is converted to a logical 1. Conversely, if the input analog signal is less than the desired threshold level, the input analog signal is converted to a logical 0. This technique of setting a threshold level for converting analog to digital is known as "data slicing."
A data slicer generally acts like a comparator. One input to the data slicer is the analog signal that is to be converted to digital. The second input to the comparator is the threshold level, also known as the "slice level." The output of the data slicer is the converted digital signal. The data slicer, in the same manner as a conventional comparator, continuously compares the analog input signal to the threshold level. When the input analog signal exceeds the desired threshold level, the data slicer outputs a constant voltage level representing a logical 1. When the input analog signal is less than the desired threshold level, the data slicer outputs no voltage, thus representing a logical 0.
A key aspect of data slicing is to set the proper slice level. A slice level that is too low will react to noise and other disturbances that are not valid data. A slice level that is too high may misinterpret actual data as no signal.
In a known method of data slicing a closed-caption signal, the slice level is automatically adjusted based on the duty cycle of the clock-run-in signal. First, a composite TV signal (of analog form) is fed to a caption signal detecting means where the closed-caption signal is extracted from the composite TV signal. The closed-caption signal is then processed by a clock run-in signal extracting means for picking up a clock run-in signal. The clock run-in signal is examined by a duty detecting means to measure a duty factor. The clock run-in signal is typically a sine wave having seven cycles, and, when it is sliced at the center of the peak to peak sine wave voltage, the duty factor can be as good as 1:1. If the slice level is higher, the duty factor may shift to 0.9:1.1, for example. If the slice level is lower, the duty factor may shift to 1.1:0.9, for example. A slice level varying means is provided for raising or lowering the slice level to the center of the peak to peak clock-run-in signal (a sine wave) so that the duty factor shifts back to 1:1, and accordingly the closed-caption signal is sliced at its optimum slice level. A data sliced form of the closed-caption signal is then decoded by a decoding means for display as a caption on a TV receiver.
The known data slicing method described above is still responsive to unwanted interference components (i.e., noise) mixed in with the closed caption signal, thereby decreasing the quality of the TV signal. Additionally, in areas where the received TV signals may be low, the slice level tends to vary abruptly, thus causing a fault action from the closed caption decoder.
Accordingly, none of the prior art devices provides a data slicer and method having the advantage of being effective, simple, and inexpensive to manufacture and implement over a wide range of operating conditions. Further, none of the prior art devices provides a data slicer and method having the advantage of being substantially unresponsive to noise in the closed caption signal, and having the additional advantage of being capable of receiving and decoding low level TV signals.
The disclosed circuit and method has the advantage of using the start bit and the parity bit to detect the presence of the closed caption signal, and thus avoids the problems associated with using, for example, the clock-run-in signal to detect the presence of the closed caption signal. The disclosed circuit and method also uses a method of calculating a desired slice level based on the minimum and maximum allowed slice level, and thus avoids the problems associated with using, for example, the duty cycle of the sine wave clock-run-in signal. The conventional method of adjusting the slice level based on the duty factor of the clock-run-in signal is substantially responsive to any unwanted interference component (namely, noise) mixed in the closed-caption signal, thus declining the quality of a TV signal. By calculating the slice level based on the minimum and maximum allowed slice levels, the circuit and method of the present invention is better able to detect low TV signals which previously tended to generate faults in the closed caption decoder when the low level closed caption signal changed abruptly.